Require Java 1.8 to build, bump maven-plugin-plugin version

[kohlschuetter]

Previously, we required Java 1.6 to compile (a corresponding entry had to be made in ~/.m2/toolchains.xml), however the code already required try-with-resources (HelpMojo).

Bump source code compatibility to Java 1.8, and remove the JDK 1.6 requirement from Maven POM files.

Also update the maven-plugin-plugin version, which seems to fix a related problem when running with newer Maven environments.

[luontola]

That would break backward compatibility with old Maven versions: https://github.com/luontola/retrolambda/blob/v2.5.7/scripts/build.sh#L63-L69